Some minor patches to be included in Mousepad 0.5

Related to #71 (closed).

A few things that I realized yesterday after merging the latest changes:

• The preferences dialog should be modal, and for safety destroyed with parent (also for consistency with other dialogs in mousepad/mousepad-dialogs.c) closed with the last MousepadWindow (see !47 (comment 17839), !35 (comment 17882) and finally !49 (merged));
• For future use in contexts other than startup and for consistency with other dialogs in mousepad/mousepad-dialogs.c, the encoding dialog should be modal, and for safety destroyed with parent;
• The replace dialog should be destroyed with parent (but not modal, e.g. to be able to select text): this is a regression caused by !35 (merged) (fixed in !49 (merged));
• The combo box to choose the file opening mode (added in !41 (merged)) should enable wrapping to prevent increasing the dialog box size depending on translations (I have the problem in French).
• The tab menu should be updated on "modified-changed" signal as the menubar and the toolbar (an oversight in !39 (merged)).