Enhancement: Media Browser improvments

This is a proposal for a little enhanced media manager (before it maybe gets completely replaced...) and fixes some issues present in the 2.3 develop branch which are:

- Cut off toolbar buttons because of increased button padding of the new theme -> This is solved by removing the buttons to expand / collapse the tree
- To wide media source combo list when opened

This PR also includes some general media manager improvements which are:
- File Detail panel styled a bit better
- File Detail image preview not using the too small thumbnail anymore
- File Detail panel also shows image dimensions if available
- File Detail panel respects manager date and time formats to show last modified date
- File Detail panel separator line in window mode
- The media manager allows for an alternative view mode: list of files instead of thumbnail grid (this is also configurable via a new system setting modx_browser_default_viewmode which can be "grid" or "list")

I furthermore separated the styles used for the media browser in a new scss file called _browser.scss and renamed the classes used as they were not really "helpful" in identifying what they actually are.

To give a better impression of what has changed, please watch the following demos

Before:
![modx browser old](https://cloud.githubusercontent.com/assets/445501/3393131/d2cd7828-fcbe-11e3-9340-12e9ebae3db4.gif)

After:
![modx browser new](https://cloud.githubusercontent.com/assets/445501/3393134/d79c0f68-fcbe-11e3-83e4-b2e1fc2b04dd.gif)