Support <1 ping per second

[lwarneminde]

Mostly UI-related tweaks to allow for slower ping rates:

• Show 1 decimal place on the Rate up/down control
• The graph may overlap the timestamps when using low ping rates (since a minute could be less than 60 pixels); add an option to avoid overlapping the timestamps (current behaviour is default)
• Fix timeline display exception when removing unsuccessful ping targets (no response in first 10 seconds)

[lwarneminde]

Lol - you beat me to it by minutes! Will rebase & see if the graph tweaks are worth bothering with.

[bp2008]

LOL. Indeed. It does sound like you've made some useful changes though.