RevBy:    partially by dt

The instance method has been moved to ICore, so there is generally no
need to include coreimpl.h anymore. Many objects accessed through this
core instance also had their own static instance methods.

Don't display the useless "Other" category, since the categorization of
commands isn't used anywhere (candidate for removal).

Filter out accelerator ampersands, since they aren't interesting here
and reduce the effectiveness of searching.

This makes both source and log files more readable and also prevents
gdb from applying extra quoting of its own.

Requested by danimo

Simple solution after all. This idea got initially dropped early in the
process because it does not handle binary data and using x/x was
way to slow. But since we use only printable characters now thanks
to base64 encoding etc using 'p' becomes feasible again and simplifies
the process vastly as no additional communication channel is needed anymore.

Task:     - none
RevBy:    - Andre

there anymore

Since it's checkable now, "Show Sidebar" is more suitable. Also changed
the interface a bit to allow more than just toggling.

replace ExtensionSystem::PluginManager::instance()->getObject<Core::ICore>() by Core::ICore::instance()

It was getting a bit cluttered without any grouping of the different
actions. Also, the collapse and expand actions have been added, mainly
for discoverability.

on gdb startup)

Now the box will grow larger when files with long filenames are opened.

forward declaration is sufficient.

Also made sure the glossary terms aren't wrapped, since that didn't look
very nice.

For the documentation about the dependencies the screenshots should be
updated again.