Fix crash in color equalizer.

[TurboGit]

We setup the tabs using GROUP_SLIDERS(). The signal switch_page is raised calling _channel_tabs_switch_callback. In this callback we are accessing gui_date (g->param_size, etc) but this widget is setup after calling GROUP_SLIDERS() which crashes.

Connect to the signal at the end of gui_init() when all widgets are created.

[TurboGit]

For unknown reason Darktable started to crashes every ten minutes while editing. After this patch all is back to normal on my side.

[dterrahe]

Are you sure this really solves a problem? darktable.gui->reset is TRUE during gui_init and in that case the notebook callback immediately returns. So this PR should not make a difference (except for saving one signal handler call). It would also be surprising if a crash triggered in gui_init didn't happen deterministically at startup 100%.

So my suspicion is that if you had a crash before, you still have it...

[TurboGit]

I don't have the backtrace anymore, but it was clearly from gui_init() to a null widget in the callback. I'll watch if it happens again, but up to now I don't have the issue.

[TurboGit]

But looking at the code you're right (of course), so no explanation for now. Just for the record this was not when starting Darktable but it was after doing some editing (Agx, Local Contrast, Exposure...) and switching to the next image. Also I was using the synch of the last edit amongst selected image on the filmstrip. Maybe one of those have a memory corruption...