Closes microsoft#5766

## PR Checklist
* [x] Closes #9382
* [x] CLA signed. 
* [ ] Tests added/passed
* [ ] Documentation updated. 
* [ ] Schema updated.
* [ ] I've discussed this with core contributors already. 

## Detailed Description of the Pull Request / Additional comments
The selection with shift is quite broken in 1.6.

It started with #8611 that introduces cell selection on `shift+click`.
This change resulted in the following defect:
`shift+double-click`, `shift+triple-click` select only parts  of the word.
The reason for this is that the first `shift+click` establishes the selection,
while the consequent clicks simply extend it to the relevant boundary
(aka word / line boundary)

However, the logic was broken even before #8611.
For instance, `shift+triple-click` had exactly the same handicap:
`shift+double-click` was establishing the selection and the
third click was simply extending it to the line boundary.

This PR addresses the both defects in the following manner:
upon multi-click that starts new selection we establish
a new selection on every consequent click using appropriate mode
(cell/word/line) rather than trying to extend one.
For this purpose we remember the position that started the selection.

## PR Checklist
* [x] Closes #9382
* [x] CLA signed.
* [ ] Tests added/passed
* [ ] Documentation updated.
* [ ] Schema updated.
* [ ] I've discussed this with core contributors already.

## Detailed Description of the Pull Request / Additional comments
The selection with shift is quite broken in 1.6.

It started with #8611 that introduces cell selection on `shift+click`.
This change resulted in the following defect:
`shift+double-click`, `shift+triple-click` select only parts  of the word.
The reason for this is that the first `shift+click` establishes the selection,
while the consequent clicks simply extend it to the relevant boundary
(aka word / line boundary)

However, the logic was broken even before #8611.
For instance, `shift+triple-click` had exactly the same handicap:
`shift+double-click` was establishing the selection and the
third click was simply extending it to the line boundary.

This PR addresses the both defects in the following manner:
upon multi-click that starts new selection we establish
a new selection on every consequent click using appropriate mode
(cell/word/line) rather than trying to extend one.
For this purpose we remember the position that started the selection.

(cherry picked from commit 83f2a3b)

## PR Checklist
* [x] Closes #9382
* [x] CLA signed.
* [ ] Tests added/passed
* [ ] Documentation updated.
* [ ] Schema updated.
* [ ] I've discussed this with core contributors already.

## Detailed Description of the Pull Request / Additional comments
The selection with shift is quite broken in 1.6.

It started with #8611 that introduces cell selection on `shift+click`.
This change resulted in the following defect:
`shift+double-click`, `shift+triple-click` select only parts  of the word.
The reason for this is that the first `shift+click` establishes the selection,
while the consequent clicks simply extend it to the relevant boundary
(aka word / line boundary)

However, the logic was broken even before #8611.
For instance, `shift+triple-click` had exactly the same handicap:
`shift+double-click` was establishing the selection and the
third click was simply extending it to the line boundary.

This PR addresses the both defects in the following manner:
upon multi-click that starts new selection we establish
a new selection on every consequent click using appropriate mode
(cell/word/line) rather than trying to extend one.
For this purpose we remember the position that started the selection.

(cherry picked from commit 83f2a3b)