Implementation of MiscShiftArrowsToScroll as accelerators

ISSUE

MiscShiftArrowsToScroll was being handled by key_press event in TerminalWidget. It was not implemented as an accelerator.

Issue: #175 (closed)

WHAT's BEEN DONE

• As was explained in the issue thread, ThunarStandardView's implementation was followed.
• Added XfceGtkActionEntries list using <Shift>Up and <Shift>Down as the accels for 1 line up & 1 line down.
• The callback to handle key_press(s) are terminal_action_shift_scroll_(up/down). The check for value of MiscUseShiftArrowsToScroll was removed (Otherwise not working).
• TerminalWidgetClass was added with connect_accelerators & disconnect_accelerators methods.
• Properties were added to the TerminalWidget.
• Only 1 property has currently been added that is of PROP_ACCEL_GROUP. Activation of which will append the accel_group of the TerminalWidget with the (value) provided in the set_property function.
• A new function to append the accels of the terminal held by screen was added that is called whenever a new tab or new window of the terminal is created.

What's working

• The accels are working as intended.