Remove support for disabling file browser status updating

This preference was introduced as a workaround for poor performance when displaying large folders, this however should no longer be an issue.
Use Objective-C literals
Don’t disable NSQuitAlwaysKeepsWindows

This was disabled in an attempt of avoiding the “do you want to open saved documents from last session?” which appear after a crash.

It doesn’t seem to have any effect though, so just noise in the code.
Re-use download window if already visible

This is useful if a new update arrives after TextMate has downloaded the previous update but before the user relaunched TextMate.

Fixes #909.