Common text selection keys should work in TextBoxes

[profexorgeek]

A user requested that TerraScriber textboxes should support expected input text editing features. Here's a list of ones that I just tested. I pre-checked things that already work:

• Pressing home should move the caret to the beginning of the line
• Pressing end should move the caret to the end of the line
• Pressing shift+home should select from the caret to the beginning of the line
• Pressing shift+end should select from the caret to the end of the line
• Pressing arrows should move the caret
• Pressing shift+arrows should change the selection accordingly
• Pressing CTRL + A should select all text
• Pressing CTRL + arrows should jump to whitespace characters

It looks like most of these already work great. However:

CTRL + A is not implemented at all. This should be implemented

CTRL + left arrow has an "off by one" issue. It should navigate through the text string in reverse, jumping to the end of each whitespace character. Instead, it jumps to the [end + 1] and ends up just after the first character after whitespace. Repro this by creating a text box, typing a string with spaces in it, and pressing CTRL + left to navigate backwards through the sentence. Notice that the caret doesn't jump to the expected place.

[vchelaru]

• Pressing CTRL + A should select all text
• Pressing CTRL + arrows should jump to whitespace characters

[vchelaru]

Fixed by vchelaru/Gum@df24f29