Sorting channels by frequency sorts lexicographically instead of numerically

[raineth]

sdrtrunk Version
v0.6.0-beta-2

Describe the bug
When sorting the frequency list in the playlist editor, the list is sorted lexicographically by frequency instead of numerically. For example, 449MHz will be sorted before 45MHz.

To Reproduce
Steps to reproduce the behavior:

1. Create channels at 45, 449, and 450MHz.
2. Click on 'frequency' header in channel list to sort by frequency.
3. Frequencies are sorted as: 449.0, 45.0, 450.0.

Expected behavior
Frequencies are sorted as: 45.0, 449.0, 450.0