[sch/brd] Add tooltips to other gui elements

[abetusk]

Even the ones that 'should' be obvious aren't. Add tooltips that explain what each gui element is, including the undo/redo, the now new clearance and trace width, the grid size, the inches/imperial and the layer guis. Also add some way to disable tooltips, even if it's a hotkey.

Tooltips will help alleviate new user confusion.

estimate: 2
priority: medium