Make the "meeting point" arrow always visible

[Rampoina]

• Makes the meeting point arrow always visible
• Removes the alpha gradient (otherwise the start of the arrow is not visible if it's too far away)
• Makes the arrow a bit slimmer and the arrowhead bigger to improve visual readability

Fixes #238

[titiger]

I don't like the always visible arrow that much maybe you can suggest something else ? Something like slightly transparent arrow on start and less transparent on target ? maybe combined with thinner arrows ? Maybe show a screenshot(s) so We can quickly decide together if its good.

[Rampoina]

@titiger First of all, as I've discussed with you this is a usability issue. If you don't see where the arrow goes you may wonder if you set the "meeting point" at all, causing users to doubt themselves and having to re-set "meeting points". Usability > (subjective) taste

Second of all, if we're talking exclusively aesthetics the arrow is ugly, always visible or not. The prettier solution would be to use a flag like in 0ad.

Third of all, I've already sent screenshots with 2 different prototypes (one of which is the last commit) on the irc channel:

Here's an animated "meeting point" indicator I made