video screenshot: show a message in the statusbar after saving
grayscale tool: show a preview of the different filters
unify equalize histogram and adjust contrast

show a preview of the various contrast filters
fixed some memory leaks
filter grid: remove the weak pointer when no longer needed
free the image data if the operation is cancelled
adjust contrast: create the previews following the buttons order
simplified the code, added some sanity checks
adjust contrast: reduced the number of filters
bigger grayscale icon
adjust colors: reduced the height of the options box
toolbox options: use an headerbar to save some vertical space
hide the header bar buttons after activating a tool
moved the file tool buttons in the main headerbar
file tools: do not accept the changes if no filter is active
extensions list: fixed label alignment
removed an unused style