caja-zoom-control.c: label_style_set_callback to GtkStyleContext in GTK3 builds

caja-places-sidebar.c: Port caja_places_sidebar_style_set to GtkStyleContext from GtkStyle in GTK3 builds

caja-pathbar.c: GtkStyleContext is already used to style the pathbar, so removed GtkStyle variables and unneeded caja_path_bar_style_set function

 Port one variable in style_set_handler to GtkStyleContext in GTK3 builds

caja-information-panel.c: Port caja_information_panel_style_set to GtkStyleContext in GTK3 builds

caja-history-sidebar.c: port caja_history_sidebar_style_set GtkStyleContext in GTK3 builds

In GTK 3.21, the use of GtkStyle in function style_set in caja-sidebar-title.c results in a segfault if the sidebar is showing, even though it is used only when the "information" sidebar is selected. GtkStyle also appears in update_title_font  in the same file.

The second function resizes the bold headline label font in the information sidebar when either the length of the text or the width of the sidebar changes. Port it to GtkStyleContext and keep it. 

The first function (style_set) does two things: It invokes the second function when the style is set up, so port its second input variable (which seems to receive only NULL anyway) to GtkStyleContext. The rest of the function is supposed to set the font for the "more information" label, but mostly seems to block updating that font with the system font and cause the size of the font to be different between GTK 3.20 or earlier and GTK 3.21. Disable that portion entirely in GTK3 builds, as that way the font is consistant across GTK3 versions(an appropriate size in all cases tested), updates with changes in the system font, and cannot segfault in GTK 3.21. Porting it to GtkStyleContext has been tested and stops the segfaults but leaves the other two problems mentioned. Disable that portion of style_set in GTK3 and be done with it.