Introduce a concept of runtime executables that can be ignored.

Summary:
This introduces a TryIgnoreRuntimes key in taskmanagerrulesrc's Settings
group that can be used to list known runtime executables that can be
ignored when trying to identify the application owning a window by the
command line of the associated process.

A concrete example is the Frozen Bubble application. Frozen Bubble
installs a frozen-bubble.desktop with Exec=frozen-bubble, yet its
WM_CLASS is ("perl", "perl") and its command line is as follows:

  /usr/bin/perl /usr/bin/frozen-bubble

The existing servicesFromPid() (now split into servicesFromPid() and
servicesFromCmdLine()) does not find the frozen-bubble service because
it works with the leading executable in the command line.

In this patch, TryIgnoreRuntimes is introduced with a default of
"perl". In the new code, after the initial command line matching pass
fails, TryIgnoreRuntimes is checked for whether the leading command
line executable is on the list, and if so, a second command line
matching pass is run with the remainder of the process' command line,
now succeeding.

Note that the approach of "try one more thing" rather than checking
against TryIgnoreRuntimes first is deliberate: The Exec= key in many
.desktop files contains a complete command line prefixed with the
runtime executable. In those cases we don't want to ignore the runtime
executable, or matching would fail.

To complement this, the "perl" WM_CLASS is added to the
MatchCommandLineFirst rc key. Due to the shoddy metadata Frozen
Bubble already ended up in servicesFromPid() regardless, but this
saves some busywork for this WM_CLASS, which can reliably be presumed
bad.

The patch also fixes a small logic error in the match-command-line-
without-arguments block, where the result of a KServiceTypeTrader
query was ignored and immediately overridden by another.

Reviewers: #plasma, davidedmundson, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5522