On mako, image#89, once the toolbar is hidden it can't be opened again

[Leo Arias]

Starting with image#89 and the qtdeclarative5-ubuntu-ui-toolkit-plugin, the gallery toolbar gets locked once it's hidden.

[Tim Peeters (tpeeters)]

some testing showed me:
- on desktop with uitk staging and gallery-app trunk, it works fine (toolbar shows on mouse hover near the bottom edge)
- on device (nexus4), other apps (notes, file-manager, browser) still work as expected

[Tim Peeters (tpeeters)]

when switching to another app, and then switching back to the first app, the toolbar should be shown again. This behavior works in notes, file-manager, but not in gallery-app.