Support proper regex in filtering rules

[fwsmit]

Dunst currently uses fnmatch(3) for pattern matching in filtering rules. It supports wildcard matching ("*") and on GNU systems you can enable some extended patterns like @(one|two|three) for matching any of a few strings or !(one|two|three) for not matching a few strings. We currently don't enable these extended patterns. It was enabled in #659, but it was decided that fnmatch is not a good solution. The man page also says that it's meant for matching file names, not generic strings.

Compatibility: supporting full regex might pose some minor compatibility problems as non-special characters might become special characters. If that is deemed a problem, a toggle can be implemented for enabling regex matching.

Fixes: #1013
Fixes: #645
Fixes: #658

Related: #659

[fwsmit]

The c regex is described in regex(3). By default is uses the POSIX Basic Regular Expression syntax and can be extended to the POSIX Extended Regular Expression syntax.

The main compatibility problem I see is the wildcard needs a character before it to repeat, so instead of * you need to use .*. So I think we need a switch to turn on this regex search for compatibility.

[fwsmit]

I've chosen the extended syntax, since it only has a few more special characters and you won't need to backlash things like | for matching multiple strings.

[fwsmit]

Implemented by #1017.