AM Demod: Add additional modes to specify channel center frequency

[srcejon]

Currently AM Demod channel center frequency is specified by Δf setting. This PR adds a couple of additional modes, that are selected by a combo box that replaces the Δf label:

Δf option uses current behaviour

25k - Allows entry of an absolute frequency specified in MHz (as is typical in the airband). This I think has two advantages:

• No need for any mental arithmetic - you can just type in the ATC frequency you are interested in.
• The demod stays locked to that frequency, even if the device center frequency changes. (So, you can make adjustments to the device center frequency, without needing to manually retune the demod)

The valuedial only displays 6 digits, as this is typically how airband frequencies are specified / spoken. Although not normally needed with AM, it's still possible to fine tune the demod center frequency to a higher precision than is displayed in the value dial, by dragging the channel marker. The precise frequency is shown at the bottom of the demod, as is currently the case.

By right clicking on the combo box, it's possible to enable snapping (combobox will be highlighted when snapping is enabled). This snaps the channel marker to 25kHz boundaries - this makes it much easier to tune accurately, when you have a wide bandwidth. For example, when bandwidth is >~2M (because you want to see activity across the spectrum), it's quite hard to accurately place the channel marker, as it's quite narrow.

8.33k - Allows entry of an 8.33kHz channel number, which is mapped to the appropriate frequency. E.g. 118.910 is mapped to 118,908,333Hz. This also supports snapping, but to 8.333kHz boundaries.

Also added a MW option, which allows specifying an absolute frequency in kHz.

Any other use cases you think might be useful?

If you like it, it could be something we added to some of the other demods too (i.e. being able to specify absolute frequencies + support snapping), where appropriate.

Patch also fixes a bug in ValueDialZ, that was preventing using the keyboard to enter the first digit, when m_positiveOnly was true.