Unified add layer dialog

[m-kuhn]

Missing functionality should be put in the browser, the individual buttons removed.

[DelazJ]

In this case, browser should also be activated by default (and even be the default panel shown to the user when QGIS is opened)

[m-kuhn]

And a single and easy to locate "add layer" button be shown to bring the browser to the front.

[SrNetoChan]

The browser can "waste" too much space when you are not using it, but at the same time be too small to work with... I'm with @m-kuhn, we need a button either to toggle it on and off, or to open it in front of QGIS's main window.

[anitagraser]

Related existing Redmine ticket: http://hub.qgis.org/issues/9451

[shenriod]

A Gdoc has been created here to collect ideas on how to implement this idea: https://docs.google.com/document/d/1aMX9jOfl10q8oETRzOHSHnRpHCc7UJ1dDnCCq3VyTAY/edit?usp=sharing

[anitagraser]

A first version is now implemented in master. Thanks to everyone involved in structuring ideas and requirements!