Consistency in StatusBar UI

Suggestions by @redmunds on https://github.com/adobe/brackets/issues/8150. Also issue #8627.

Add tooltip and underline text when hovering over the OVR/INS element. Change cursor to hand when hovering over the Lint icon.
Actually, setting tooltip needs to be done *here* in `_initOverwriteMode()` and not in `_updateOverwriteLabel()`.