add seperate width update interval

The `indicator-multiload` updates its width at every update interval of the values displayed as numbers (e.g. `Disk 0.0 kB/s/1.0 kB/s`). This causes the width of the indicator to change and all items left of the indicator in the bar to be moved to the left and back. This is disturbing when updates occur every 500 ms.

Introducing an update interval for the width (independent from the update interval of values) with a default value of 5 s or 1 minute would help. A simpler alternative would be to enlarge the indicator only and change back the width to a minimum at all.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: indicator-multiload 0.4-0ubuntu3
Uname: Linux 4.0.0-040000-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Apr 22 10:16:33 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (19 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: indicator-multiload
UpgradeStatus: No upgrade log present (probably fresh install)