ui: Fix GUI initialization order

Fixes at least #3478.

Splits and documents the phases:
1. Parse command-line options. These take precedence over anything else.
2. Basic Qt initialization (not dependent on parameters or configuration)
3. Application identification
4. Initialization of translations
5. Now that settings and translations are available, ask user for data directory
6. Determine availability of data directory and parse bitcoin.conf
7. URI IPC sending
8. Main GUI initialization

Splits command line parsing logic from ipcSendCommandLine into
ipcParseCommandLine, as isTestNet() can only be overridden in the early
stages before choosing a data directory. Sending however needs to happen
after choosing a data directory.
Upgrade gitian win32 to boost-1.55.

Fixes issue where all network activity just stops.
gitian: Add -O2 for win32 build

In the win32 gitian descriptor the C*FLAGS are being overridden on
`configure`, which causes the built-in optimization flags to be removed.
Add `-O2` manually (but not `-g` as we're not doing anything with the
debug information).

Improves performance of win32 gitian builds.

As a bonus, fixes issue #3497.
Merge pull request #3503

0d512a9 gitian: Add -O2 for win32 build (Wladimir J. van der Laan)
Merge pull request #3501

56fce1f Upgrade gitian win32 to boost-1.55. (Warren Togami)
Merge pull request #3488

2102ab9 ui: Fix GUI initialization order (Wladimir J. van der Laan)