Fix some issue with the client preview for the tag and task preview widgets

[Kasper24]

@javacafe01

Some other issues:

1. I think these widgets should be turned into proper objects, with methods like :show() :hide() and :toggle() instead of the current signals.
2. When there isn't an available client.content to preview, the tag preview displays a black background with the client icon in the cener. The task preview before this PR would always show the client.content, so there wasn't a need for that - but maybe we should at now do it here too?
3. The task preview height differes between clients. I think it has to be with the title not being limited in size?
4. What about using Add a new 'icon-theme' module #129 to the get the icons for the running clients? It will retrive the icon for the running client from the currently system selected icon theme, or whatever theme was passed to the constructor

[javacafe01]

@Kasper24

1. I think this is a great idea. I believe Bling is overusing signals 😄. I think we can wait for - #99 to finish or add this to the list for that PR.

2. The client icon is already shown in the preview, so I think if there is nothing to show we just don't show an image (just the icon and title), or we can show the AwesomeWM logo as a fallback.

3. That, and maybe also because the client previews are different sizes?

4. Seems like a good idea! But for a different PR, not this one.