Fix hang on broken project files, empty settings files, row count wrap-around and plot button style - #4248

Project files:

The loops which read the elements of a project file used the condition
"readNext() != EndElement && name != <element>". Once the XML reader hits
an error, e.g. because the file is cut off or contains invalid XML, it
only returns Invalid tokens from then on, so these loops never ended. The
application then spun at 100% CPU while opening such a project file,
before even showing its window.

Stop the loops when the reader has reached the end of the document or
failed, which cannot happen for well-formed input before the element was
closed. The elements which have been read up to there are still restored
and the user is now told that the project file could not be read
completely.

Extend the global filter test with broken documents: it runs
GlobalFilter::load() on a worker thread with a time limit, so that a loop
which does not end fails the test instead of hanging it.

Settings files:

The -S/--settings option and the import of settings only accepted a file
if its first line is "[%General]". A file which already exists but is
still empty, e.g. one which has been created in advance for a portable
installation, has no first line at all and was rejected as "not a normal
settings file", so the option was ignored and the settings ended up in the
registry or the user's configuration instead. Treat an empty file like
one which doesn't exist yet. Files with other content are still rejected.

Add a test for the settings file handling.

Related: sqlitebrowser#2791

The button is meant to turn red when not all data has been fetched yet,
to hint that the plot is incomplete. Its style sheet only set the colours
but no border, so the button was still drawn by the style of the
application, which then has to take the background colour from the
palette. Some platform themes and styles don't do that (QT_QPA_PLATFORMTHEME
set to qt5ct or gtk2 for example), and the button stayed in the normal
colours.

Also set a border in the style sheet. This makes Qt draw the complete
button from the style sheet, whatever the style of the application is.

Add a test which shows the plot of a partially fetched table and checks
that the button is red with the Fusion and Windows styles as well as with
a style that ignores the palette (a stand-in for the platform themes),
and that it is not red when all data has been fetched.

Related: sqlitebrowser#2001