[bug 914122] Don't use letter-spacing on grid icons, as it results in poor glyph spacing

See bug 914122 comment 40. Using negative letter-spacing gives poor-looking text, because the glyph positions are rounded to whole pixels and therefore fine adjustments to the individual positions cannot be accurately realized; instead, we get a few abrupt, ugly whole-pixel shifts at "random" places within the string.
Bug 934011 - [music] Force closing the Music app breaks the controls
Bug 939047 - [B2G][Helix][camera][dingyu]The camera preview is not correct and the left of the screen have a black area in hd device
Bug 938707 - Fix failure of test_settings_do_not_track
Bug 938931 - [FTU] Fix checkbox regressions from bug #914892
Bug 937067 - Add compatibility with new async method availableNetworks of NetworkStats API. r=salva
Bug 939149 - [Action Icons - FM Radio] Remove shadows from action icons
Bug 939148 - [Action Icons - Calendar] Remove shadows from action icons
Merge pull request #13738 from huchengtw-moz/camera/Bug_939047_black_border_on_left_side

Bug 939047 - [B2G][Helix][camera][dingyu]The camera preview is not corre..., r=djf
Bug 923297 - [B2G][Video] Video duration can appear off by a second when played in Video app
Merge pull request #12960 from huchengtw-moz/vide/Bug_923297_video_duration_mismatch

Bug 923297 - [B2G][Video] Video duration can appear off by a second when..., r=djf
Bug 929318 - [HW test] Radio test
Merge pull request #13717 from jimporter/music-iac-restart

Bug 934011 - [music] Force closing the Music app breaks the controls; r=alive
Merge pull request #13727 from MBRSL/bug-929318-radio

Bug 929318 - [HW test] Radio test, r=gasolin
Merge pull request #13591 from acperez/bug-937067

Bug 937067 - Add compatibility with new async method availableNetworks of NetworkStats API. r=salva
Merge pull request #13785 from snowmantw/issue939434

Bug 939434 - Lock screen handle does not return to its original position when you hit "cancel" from passcode panel, r=timdream
Bug 930544 - Allow installing applications on several empty screens
Bug 923024 - [MMS] Add settings button on top bar

--------------------
- Create options button
- Call OptionsMenu
- Settings Activity
- Added tests
Bug 939729 - With httpd.js we sometimes don't get the most recent version of the file r=vingtetun

This patch:
* makes all files expire immediately, forcing a revalidation by Firefox
* implements conditional requests with Last-Modified-Since
Bug 930104 - Remove localId=0 hack for the system app on firefox and correctly set permission for any app loaded in Firefox r=vingtetun