[EGD-5697] Framework for phone modes introduced

It allows to transmit phone modes information to listening services.

[EGD-5324] Add Low Power documentation

Documentation describing the Low Power control algorithm and
synchronization with the system using dedicated sentinels and drivers.

[EGD-4797] Battery bar as widget

Battery bar now is present as
a separate widget in top bar.
Additionally refactored passing
battery status.

[EGD-5382] Add LowPower CpuSentinels

In order to synchronize the Low Power mode, the services were
immediately informed about the frequency change,
so that they can update their resources (e.g. PWM filling)
and services may request the maximum CPU frequency in order
to perform a task (e.g. screen redraw, telephone conversation)

[EGD-4068] Add Update Package Installation Mode

It is necessary that the PureOS update mechanism enters Update Package
Installation Mode (where most of the services are stopped)
when the PureOS update package gets downloaded into the phone.

[EGD-5027] Services synchronization introduced

Dependencies between services are now supported.

[EGD-5381] Add Low Power Drivers

Peripherals clock synchronization dependent on PLL2

[EGD-5158] Change access to the bus methods

The bus is now accessible only via Service object.

[EGD-3247] Remove reminders of unused ping-pong feature

Feature was already removed some time ago but still some reminders
persist. This commit only removed mentioned reminders.
Some minor style fixes suggested by clang tidy were also applied.

[EGD-5090] Add Brownout GUI window

Gui window will appear due to brownout
event. System shutdown will be also called.

Merge branch 'master' into stable

[EGD-5325] Remove using namespace directives from headers

"Using namespace" directive should not be added in headers
because this practice pollutes source files.

Merge commit 'c7dd6b399' into stable

Merge branch 'master' into stable

[EGD-2501] Add gsm powerdown due to battery

Gsm model power state maintenance based on battery level.

[EGD-5186] Change minimum CPU frequency to 132 MHz

Temporary minimum CPU frequency limitation

[EGD-4887] Add PLL2 clock switching

Pll2 will be disabled at the lowest CPU frequency,
which will save power

[EGD-4881] Add oscillator clock switching mechanisms

Osc will change automatically at the lowest CPU frequency,
which will save power

[EGD-5050] Add brownout detection

Brownout detectection realized through charger driver

[EGD-4694] Add CPU frequency shift mechanism

The CPU frequency changes automatically which saves energy