CImage: Fix null pointer errors.

Now that 'null' is an allowed value for m_bitmap, we have
to check for it in all functions to avoid crashing.
CNumericEdit: Adjust auto-increment times.

The new values have increased delay before starting to auto-increment.
This avoids the situation where a slightly long click could cause
multiple increments to happen.
CTaskbar: Add redraw() method.

Usually redraw() is not necessary, but if one changes the display
state manually, it is nice to be able to redraw the whole display.
The taskbar is the most logical place to do this.
Add new function stm32_dmacapable().

This allows to check whether given memory region can be used
for DMA transfers. Currently it is important only for
excluding the STM32F4xx 64kB CCM RAM block.
stm32_spi: Fall back to non-DMA transfers when memory region is unsupported.

This allows spi transfers to function for stack allocated buffers
even when the stack is in CCM. Naturally the non-DMA transfer is
going to be slower, but it doesn't matter that much for small transfers,
such as with the touchscreen drivers.