Update KIPI toolbar button after installation of kipi-plugins

Summary:
In case there are no KIPI plugins installed, Gwenview can offer to
{nav Install Plugins}. After installation completed, the {nav Plugins}
entry in the menubar would update itself, however the KIPI toolbar
button would still show {nav No Plugin found}.

By always listening to `KIPIInterface::loadingFinished`, we are notified
through `mPluginLoadTimer` when the menu needs to be updated in reaction
to the plugin installation finishing.

To avoid showing {nav Install Plugins} as a {nav Last Used Plugin} in
case the installation was aborted, that section is only added when it
makes sense, i.e. when there is actually more than one plugin to show.

While `mExportActionList` does not contain {nav Install Plugins} anymore
after installation completed successfully, it is still contained in
`mDefaultAction`, which cannot be prevented easily due to not having
access to `KIPIInterfacePrivate::mInstallPluginAction`. Therefore we
only ever add actions from `mExportActionList`, which is the right thing
to do in any case.

Test Plan:
Install {nav Discover}, remove `kipi-plugins`, open Gwenview and
click on {nav Plugins > Install Plugins}. Keep Gwenview open while
installing `kipi-plugins` and observe both the menu entry and the
toolbar button populating themselves after installation completed.

Restarting Gwenview afterwards should populate the toolbar button like
normal.

Reviewers: #gwenview, faridb

Reviewed By: faridb

Differential Revision: https://phabricator.kde.org/D13252