Fixed type testing of media items (qgraphicsitem_cast couldn't tell the difference between audio and video items)

The scale of PDF items was sometimes badly calculated when opening a
document made with a previous version of OpenBoard or made on another
computer.

Specifically, this solves the following issues:

- PDF scale calculation in documents that did not specify the pageDPI
used to render the PDF (happened with documents created with some old
versions of OpenBoard)

- PDF scale calculation in multi-page documents (it was set correctly
for the first page, but not the following ones)

In some cases, the PDF background of a document could be scaled badly
when tools such as the ruler, compass etc. were present on the page.

This happened with PDFs of version <= 1.4, and when the tools were
outside of / larger than the page.

This fixes an issue where if one document was imported with a different
DPI than the current one, any document created thereafter would have
this same value (which could then cause problems if a PDF was added to
that new document).

Saving this value to UBDocumentProxy not only makes more sense, it also
fixes this issue.

Brazilian portuguese translation

Fix for window being placed badly on the screen when returning to board mode from desktop mode on some environments

Caused problems e.g with podcast mode, where if the control and display
views were swapped in the preferences, the wrong screen would be recorded
when switching to desktop mode during recording of the podcast.
This happened even if only one screen was plugged in, so a black screen
was recorded in that case (at least on OS X 10.10)

(when an opque stroke was in the vicinity of a translucent one, it
became translucent itself)

    - Qt plugins and libraries (and associated files) bundled only if necessary
    - Dependencies corrected (for Ubuntu 14.04 and 16.04, at least),
    mostly manually for now
    - run.sh sets Qt plugin path environment variable (=> qt.conf
    no longer needed; removed)

Just commented out the line since I'm not 100% sure it can be removed
(though I can't find a case where it is useful)

- Separated building and packaging
- OpenBoard is now installed in /opt/openboard instead of
/usr/local/OpenBoard-$VERSION/
- Placed a shortcut to run.sh in /usr/bin (so it can be executed with
the `openboard` command)
- Corrected .deb control files to avoid warnings e.g when installing via
software center on Ubuntu
- Added xdg bindings so that .ubz files are recognized and opened
automatically with OpenBoard

- Moved the release scripts into their own folder to clean up the
repository root a bit

(Merge branch 'new-release-script-linux' into dev)