Bug 1442419 - Fix failure to open files after downloading with snap r=jlorenzo

This fix does several things:
 * Removes the mime cache generated by the desktop-gtk3 remote part
 * Installs a stub shared-mime-info database
 * Set default association for all types to use xdg-open

Note: There is still work[1] to be completed in snapd, adding OpenFile
support to xdg-open.  Landing this is harmless though, it will fail
silently just as it does today but will start working when the snapd
feature lands.

1. https://github.com/snapcore/snapd/pull/4766
opening requested files,

MozReview-Commit-ID: 1eeOLeVN8xQ

--HG--
extra : rebase_source : e948bafc583f91177530ce00ec26c69619a7f8f9

taskcluster/docker/firefox-snap/mime-handler.xml

@@ -0,0 +1,7 @@
+<mime-info
+    xmlns="http://www.freedesktop.org/standards/shared-mime-info">
+  <mime-type type="application/octet-stream">
+    <comment>Unknown</comment>
+    <glob pattern="*"/>
+  </mime-type>
+</mime-info>

taskcluster/docker/firefox-snap/mimeapps.list

@@ -0,0 +1,4 @@
+[Default Applications]
+application/octet-stream=xdg-open.desktop
+text/plain=xdg-open.desktop
+inode/directory=xdg-open.desktop

taskcluster/docker/firefox-snap/snapcraft.yaml.in

@@ -36,6 +36,10 @@ plugs:
     allow-sandbox: true
 
 parts:
+  desktop-gtk3:
+    prime:
+      - -usr/share/mime
+
   firefox:
     plugin: dump
     source: source
@@ -52,11 +56,30 @@ parts:
       - ffmpeg
     after: [desktop-gtk3]
 
-  optimization:
+  xdg-open:
     after: [firefox]
     plugin: nil
-    stage-packages:
-      - shared-mime-info
+    source: .
     install: |
       set -eux
+      mkdir -p $SNAPCRAFT_PART_INSTALL/usr/share/applications
+      install -m 644 mimeapps.list $SNAPCRAFT_PART_INSTALL/usr/share/applications
+      update-desktop-database -v $SNAPCRAFT_PART_INSTALL/usr/share/applications
+    build-packages:
+      - desktop-file-utils
+    build-attributes: [no-system-libraries]
+
+  shared-mime-info:
+    after: [xdg-open]
+    plugin: nil
+    source: .
+    build-packages:
+      - shared-mime-info
+    build-attributes: [no-system-libraries]
+    install: |
+      set -eux
+      mkdir -p $SNAPCRAFT_PART_INSTALL/usr/share/mime/packages
+      install -m 644 mime-handler.xml $SNAPCRAFT_PART_INSTALL/usr/share/mime/packages
       update-mime-database $SNAPCRAFT_PART_INSTALL/usr/share/mime
+    stage:
+      - usr/share/mime