use QWebView to show the user manual

Looks like the QTextBrowser can't render the manual correctly.
Also, QWebView provides a better way to find contents on a webpage,
which is an important feature for an user manual (to be implemented).

Signed-off-by: Danilo Cesar Lemes de Paula <danilo.eu@gmail.com>
fixing planner's gas dialog

Before this commit this dialog had a weird behavior in which
when it was shown, the whole screen was still active. In case of a
alt+tab, this dialog is sent to behind the application window, but still
visible in the task bar.

This commit basically changes this behavior to a regular pop-up.

Signed-off-by: Danilo Cesar Lemes de Paula <danilo.cesar@collabora.co.uk>
[PATCH] fix DownloadDialog behavior

The DownloadDialog behavior was broken in a way it allows the user
to make changes on the dialog while the download is happening.

Also, clicking on "Cancel" breaks/hangs the UI sometimes, as libdivingcomputer
doesn't always cancels the download right away. That's a bug that
still needs to be fixed.

Signed-off-by: Danilo Cesar Lemes de Paula <danilo.eu@gmail.com>
WIP