Set Tk selection anchor when using Home/End keys

[windymilla]

When used without Shift, Home/End (Cmd-Left/Right on Macs) should set the selection anchor so that future adjustment of the selection works more predictably.

Fixes #1920

[tangledhelix]

This still does the wrong thing when (on Mac only) you move the cursor to start/end of line with Ctrl-A (start) or Ctrl-E (end).

[tangledhelix]

Strangely, though, it does work if you don't involve arrow keys in it!

Works:

• Click in middle of some line
• Ctrl-A to jump to start of line
• Shift-Ctrl-E to select to end of line
• Shift-Ctrl-B to move backward 1 char (selection is now entire line minus last character)

(The same is true in reverse: Ctrl-E, Shift-Ctrl-A, Shift-Ctrl-F.)

Doesn't work:

• Click in middle of some line
• Ctrl-A to jump to start of line
• Shift-Cmd-Right to select to end of line
• Shift-Cmd-Left to move backward 1 char

This leaves the selection from where you clicked until just before the last char in the line.

[srjfoo]

Doesn't work:

• Click in middle of some line
• Ctrl-A to jump to start of line
• Shift-Cmd-Right to select to end of line
• Shift-Cmd-Left to move backward 1 char

This leaves the selection from where you clicked until just before the last char in the line.

For the last bullet point, Shift-Cmd-Left should take you back to the beginning of the line. Did you mean Shift-Left?

[tangledhelix]

Yes