gui.Edit: Added possibility to select text

[countcb]

• shift + left/right expands/shrinks selection to left/right

• shift + home/end expands selection to beginning/end of text

• ctrl + a selects everything

• shift + delete removes everything (even when nothing is selected)

• selected text can be deleted with delete or backspace

• selected text can be overwritten by inputting new text while text is selected

• selected text is highlighted even when the edit box has no focus

• SelectedText() method to get only the selected text.

• everything should mimic a "normal" text field (so everything should behave like expected)

• mouse selection working now

[BenoitBotton]

HI, Any idea on when this might be merged? can I help in testing it?

[danaugrs]

Thanks a lot @countcb ! Your implementation is top notch. Sorry for the delay in merging this.

[countcb]

Sweet, thanks for the merge! Glad I could help :)