Prepare to release version 3.4. Helps to fix Issue #95.

Hi!


<del>Please __do not stash the `3.5pre` version bump commit__ with the others! 
Merge this pull request as 2 commit: one for the release notes and one for version bump.</del>

There should be a commit where the `<version>` is `3.4` in `install.rdf`!
That commit should be tagged as `v3.4`!


Release Notes contains:
 * Include changeset info in the Insert/Copy Build ID menuitem and in 'Customize Titlebar'. Issue #65 / Pull #73.
 * Add pushlog-to-tip menuitem.  Issue #61 / Pull #75.
 * Open customize.xul (Customize Titlebar) as a resizable window. Issue #55 / Pull #79.
 * Let nightlyApp.openNotification() fallback to notificationBox in legacy Fx. Issue #81 / Pull #83.
 * Include about:nightly in Nightly Tester Tools' menupopup for Thunderbird. #56 / Pull #76.
 * Don't call callback function when iterating over Extension Manager's extensions list. Issue #88 / Pull #89.
 * Don't register aboutNightly component for 'profile-after-change' notification. Issue #57 / Pull #77.
 * Update 'Contributors' section automatically in about:nightly. Issue #58 / Pull #85.
 * Move to MPL2. Issue #39 / Pull #96, #98.
 * Column resizing in 'Customize Titlebar' window. Issue #25 / Pull #74.
 * Add privacy context to saveScreenshot() due to Bug 795065. Issue #99 / Pull #102.
 * Update checkCompatibility preferences for compatibility. Pull #103.
 * Use GreD instead CurProcD to reference GRE specific files. Issue #115 / Pull #116.

Nice to have (but not blocking!) fixes (in priority order):
 * Broken upload APIs: screenshot (Issue #49 / Pull #84) and `about:support` (Issue #86 / Pull #93)
 * ... and the others ;)

Please note that the issue numbers in the commit are the __issue__ numbers instead __pull__ numbers!


__AMO Release Notes strings__:

	Built from <a href="https://github.com/mozilla/nightlytt/commit/99f1679e01fc"><code>99f1679e01fc</code></a>.
	<ul>
	  <li>Extension: Use GreD instead CurProcD to reference GRE specific files. (<a href="https://github.com/mozilla/nightlytt/issues/115">#115</a>)</li>
	  <li>Extension: Update checkCompatibility preferences for compatibility. (<a href="https://github.com/mozilla/nightlytt/issues/103">#103</a>)</li>
	  <li>Extension: Add privacy context to <code>saveScreenshot()</code> due to <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=795065">Bug 795065</a>. (<a href="https://github.com/mozilla/nightlytt/issues/99">#99</a>)</li>
	  <li>Extension: Make columns of 'Customize Titlebar' dialog's tree resizable. (<a href="https://github.com/mozilla/nightlytt/issues/25">#25</a>)</li>
	  <li>Extension: Re-license under MPL2. (<a href="https://github.com/mozilla/nightlytt/issues/39">#39</a>, <a href="https://github.com/mozilla/nightlytt/issues/98">#98</a>)</li>
	  <li>Extension: Update 'Contributors' section automatically in about:nightly. (<a href="https://github.com/mozilla/nightlytt/issues/58">#58</a>)</li>
	  <li>Extension: Don't register aboutNightly component for 'profile-after-change' notification. (<a href="https://github.com/mozilla/nightlytt/issues/57">#57</a>)</li>
	  <li>Extension: Don't call callback function when iterating over Extension Manager's extensions list. (<a href="https://github.com/mozilla/nightlytt/issues/88">#88</a>)</li>
	  <li>Extension: Include about:nightly in Nightly Tester Tools' menupopup for Thunderbird. (<a href="https://github.com/mozilla/nightlytt/issues/56">#56</a>)</li>
	  <li>Extension: Let <code>nightlyApp.openNotification()</code> fallback to notificationBox in legacy Fx. (<a href="https://github.com/mozilla/nightlytt/issues/81">#81</a>)</li>
	  <li>Extension: Open customize.xul (Customize Titlebar)</li> as a resizable window. (<a href="https://github.com/mozilla/nightlytt/issues/55">#55</a>)</li>
	  <li>Extension: Add pushlog-to-tip menuitem. (<a href="https://github.com/mozilla/nightlytt/issues/61">#61</a>)</li>
	  <li>Extension: Include changeset info in the Insert/Copy Build ID menuitem and in 'Customize Titlebar'. (<a href="https://github.com/mozilla/nightlytt/issues/65">#65</a>)</li>
	</ul>
	
	Details are on <a href="https://github.com/mozilla/nightlytt/compare/v3.3...v3.4">GitHub</a>.