UI Polish

What's new:

* Added modal window animation to all modal windows
* Added drop-down animation to all drop-down menus except for code-hint menus
* Added Quick Open drop-down animation
* Made Extension Manager keyword text quieter to reduce noise
* Added input focus/error ring transition for all input fields
* Refactored and added pressed state for all quiet dark button in the side bar (e.g. Project, Working Set Options drop-down menu. Brackets Git extension can also benefit from this)
* Replaced Extension Search clear icon to make sure we're using the same "X" in our UI
* Replaced Side Bar selection background image with CSS
* Made Extension Manager alert text much smaller as they can take up a lot of space
* Refined Topcoat. The lines are finer now with subtler highlights and shadows; focus and error rings on controls are also on a sweet spot now. 
* Replaced double triangle with single triangle for #7134
* Removed unused icons.