rfe: make wx vdigit a bit more self-documenting

[hamish]

Hi,

in the wxGUI vdigit tool, it's not clear what mouse buttons you have to click to make things happen.

perhaps the tooltips for each button could have another sentence of text to them describing "left click to set point, right click to confirm, middle drag to pan" or so? or perhaops pack the mouse button instructions in the bottom status bar row with the map coords somehow?

Also it would be nice to mention the keyboard shortcuts in the tooltip too (see also #497) :)

(I'm all in favour of multi-sentence insta-doc tooltips everywhere)

thanks, Hamish