Bug 973992 - Support experiments add-ons; r=Unfocused

A new type of add-on is introduced: experiments.

Experiments are add-ons that conduct data experiments. They are deployed
to compatible clients and collect additional information about the
client.

--HG--
extra : rebase_source : 98a612933978b3e4bcfb83da9f6a9782c62dc0da
extra : amend_source : 9347a56bc3034c7ea1e50918278c6b9cc1270521
Bug 973992 - Telemetry experiments: Make the experiments code use the new experiments addon type. r=Unfocused

--HG--
extra : rebase_source : 54de4767390441a895fca148c2c79483f22c1366
Bug 986040 - Telemetry experiments: Assure that no experiment addon is running yet before starting an experiment. r=Unfocused

--HG--
extra : rebase_source : 55be48f0b9cffe978c22baa8260b161f2cbc0562
Merge m-c to fx-team
Bug 879921 - Use the "tabbable" class to make the new tab page's close button tabbable. r=Gijs
Bug 879921 - Replace GTK close icons on Linux with our own except for on menus and dialog buttons. r=Gijs
Bug 979747 - Inherit text color on toolbars above the titlebar on Windows Classic. f=Gijs r=mikedeboer
[Australis]
Bug 988305 - UITour: getTarget should catch exceptions from a target's query function. r=Unfocused
Bug 989105 - Redirect Send Tab to Status activity when the Firefox Account definitely cannot send a tab. r=rnewman

--HG--
extra : rebase_source : fa924a2d14147041604c67758270c85603a43acb
Bug 988571 - Migrate Android Firefox Accounts from Doghouse to Separated state on application upgrade. r=rnewman
Bug 987890 - Hide unused tools when using addon debugger. r=jryans
Bug 988661 - Add 'Enable addon debugging' button in toolbox. r=jryans
Bug 985206 - Insert colon after completing a CSS property name in the style editor. r=jwalker  f=Optimizer
Bug 972503 - Allow home panel views to specify an initial filter. r=lucasr
Bug 946595 - High contrast themes on Windows 8 shouldn't be considered the default theme in CSS, r=jimm
Bug 987461: prevent bookmark menu button dropmarker icon to stretch when placed in certain toolbars. r=Gijs
Bug 933739 - Reset autocompletion state when the toolbar gets focus (r=bnicholson)
Bug 880470 - Fix a reference from the undefined "self" to "this". r=dcamp
Bug 983951 - Check selective set of preferences instead of all prefs. r=jryans
Bug 988027 - Add geo.wifi.uri pref to the webapp runtime (Use the Mozilla Location Service). r=dougt