Disabling save to presets

[LukasT.dev@gmail.com]

Hello, 

I would like to ask you for review for this change as I feel I need a review.

I fixed the custom brush dialog and the presets that are produced when the 
"brush tip" is in "custom brush" dialog mode are useless to be saved.

The solution for this is to disable the button for saving the presets I think.
We probably do not allow to save the preset in other place, right?

Internally the preset has to be created, because otherwise the painting would
not work. But the preset is working only with Krita running as temporary brush
is inserted into brush resource server.

I added new virtual method bool presetIsValid() to KisPaintOpSettingsWidget
and when the user click on Custom brush, it will return false. So the whole 
patch is just because of Custom brush. 

That arise a little question: should be the editor for creating the brushes 
part of the dialog? Should be or not should be? That's the question...

The status of the button is updated every time user changes the current preset 
and also when the paintop is changed.

Thanks goes to slangkamp, he proposed some changes that are already in the 
patch, he did the review and gave me ship it!

What do you think about the solution? Do you think I miss same case to 
disable/enable the button? I did tests and I did not find any so far :)

Here is the patch
http://lukast.mediablog.sk/patches/disable-save-button-for-custom-brush.patch

Lukas